Genes and Heredity

Update:

• UNIT TEST ON THURSDAY.– Chapters 11, 12, 13

• Suggested Review:

Page 514, #1-11, 14, 16-17, 23, 25-27, 29-32.

Page 515, # 33-40, 42, 44, 49-53.

(Look over your notes!)

Theories of Heredity

• 17th century - the homunculus, a miniature human being, in human sperm cells

• 17th century - the egg contains an entire human in miniature and that semen only stimulates its growth.

Gregor Mendel (1822-1884)

- was an Austrian monk- during his childhood,

he worked as a gardener, studied beekeeping

- between 1856 and 1863 tested 29,000 pea plants

Why Garden Peas?

- He noticed that there were a number of distinct traits that were only one of two ways

Why Garden Peas?

- Secondly, pea plants can reproduce through self-fertilizing or cross-fertilizing

- Pollen produced from the male stamen (anther and filament) fertilizes the female ovary

- He cut the stamens off and used pure breeding plants only

Mendel’s Experiments

- before Mendel, people thought mixing traits would create a blend (e.g smooth peas bred with wrinkled peas produces slightly wrinkled peas)

- he took pollen from round seed plants and crossed it with the egg of a wrinkled seed plants and vice versa

- ALL THE SEEDS CAME OUT ROUND. The trait seemed to dominate the other trait

- He repeated this for other characteristics of the pea plant, and realized that one trait dominated the other

Results- Mendel hypothesized that factors controlled the traits of a plant- We now know these factors as genes – segments of DNA on a

chromosome that code for a certain trait (hair colour, eye colour etc)- Two or more alternate forms of a gene are called alleles, which are either:- Dominant, meaning they are always expressed when they are genetically

present, or they are: - Recessive, meaning they are overruled by dominant alleles, and are

expressed only when they are the sole allele present

Hybrid crosses

- When Mendel crossed a pure breeding round seed pea plant with pure breeding wrinkled seed plants, he produced hybrids, meaning each plant had one allele different from their parent for a specific trait

- The parent generation were pure breeding (RR and rr for seed shape) producing all round peas (Rr)

- He crossed the plants of this first filial generation (F1) (Rr X Rr) which make R and r gametes

- 75% were round, and 25% were wrinkled in the second filial generation (F2)

In summary

1. Inherited characteristics are controlled by factors, known as genes, that occur in pairs.

2. One factor, or allele, masks the expression of another. This is the principle of dominance.

3. A pair of factors, or alleles, separate or segregate during the formation of sex cells (meiosis). This is known as the law of segregation. Mendel predicted this without knowing about meiosis!

Patience is a Virtue

- Mendel’s paper was referenced three times in thirty five years and basically forgotten

- Darwin (1809 –1882) didn’t know of it. He believed in:

- Pangenesis, which holds that body cells shed gemmules, which enter the blood stream and collect in the reproductive organs prior to fertilization. Thus every cell in the body has a 'vote' in the constitution of the offspring

- Mendel’s work was rediscovered in 1900

4.2 – The Inheritance of One Trait

Human Genetics

• Each human cell has 46 chromosomes in 23 pairs, half donated from your mother (M) and half donated from your father, (F).

Homologous Chromosomes

• Again, one chromosome from the pair is donated from the mother, one from the father.

• Homologous: chromosomes which are the same size and shape, and code for the same traits (genes).

• How many pairs of homologous chromosomes would a woman have? A man? Why?

For a certain gene (like eye colour), the location of where it is coded for is the same on both homologous chromosomes.

B b

For a certain trait (like eye colour), there is variation (brown or blue) which we attribute as either dominant or recessive based on how strongly they are expressed.

Single-Trait Inheritance Terms

• ______________: refers to the alleles that an organism contains for a particular trait. One allele for each trait is inherited from each parent, which is why they occur in pairs.

• E.g. Tall (T): is the ___________________ for pea plants and short (t) is the ____________________ for the height gene.

• _________________: refers to the observable traits of an individual. In short, what we see (not always... Exceptions?).

Genotype vs. Phenotype

• How many genotypes for seed colour does a pea plant have?

• How many phenotypes for seed colour does a pea plant have?

• Review– All recessive PHENOTYPES must have a

____________________ genotype.– Dominant PHENOTYPES can have

___________________ or ____________________ genotypes.

Punnett Squares

• A diagram used to predict the outcome of a particular cross or breeding experiment. – Determines the PROBABILITY of an offspring

having a particular genotype.

Punnett Squares Example

• The allele for blue eyes (b) is recessive, whereas the allele for brown eyes (B) is dominant. If a homozygous brown eyed man conceives a child with a blue eyed woman, what is the probability of the child having blue eyes?

Monohybrid Cross

• Deals with ONE trait, and is the cross or breeding of TWO different, contrasting alleles.

• Eg. Tt x Tt

Mendelian Ratios

• Mendelian Ratio for Phenotype:

• Mendelian Ratio For Genotype:

Practice Problems

• Let’s do questions 1-3 on page 130!

Pedigrees

• Crosses can be done easily with plants, such as the common garden pea, in order to determine the inheritance of traits.

• Experimental crosses are not possible with humans.

• Human geneticists use medical, historical, and family records to study crosses that have already occurred.

Simple Dominant Traits

• A simple dominant trait has only two possible alleles – dominant or recessive.

• Examples of simple dominant traits: widow’s peak hairline, ability to roll the tongue, strait thumb, and detached earlobe.

Back to Pedegrees

• Squares generally used to represent males and circles represent females.

• Colour/shading represents individuals who are recessive or dominant for a single trait.

• Generations represented by roman numerals.

• What do you think a HALF SHADED square or circle means?: __________________________________

Determining Genotypes

• How can you determine if a particular organism is homozygous dominant or heterozygous?

• TEST CROSSES!

Test Cross

• When we know the dominant phenotype of an organism but we want to know it’s genotype.

• We always test it with the recessive phenotype.

Test Cross Example

• We have two parental sheep, one black and one white. White is dominant and black is recessive. How do we know the genotype of the white sheep?

• If the phenotype of the offspring were 50% white and 50% black, we would know that the unknown allele is actually the recessive allele, w. If all the offspring were white, then we would know that the unknown allele is actually the dominant allele, W.

Test Crossing to determine the Number of Alleles

• Using test crosses, we can determine whether a particular trait is passed by one, two, or more pairs of alleles.

Fun Stuff

• Page 132, Investigation.

Section Review

• Page 135, #1,2,3,4,5,6,7,10,11,12,13-16

(look in section 4.2 for answers to questions not covered in the ppt).

4.3 – The Inheritance of Two Traits

• Were there any problems with the homework?

POP QUIZ!

• On the board, define the following terms:– Dominant Allele– Recessive Allele– Homozygous– Heterozygous– Genotype– Phenotype– Trait– Homologous Chromosomes

RECAP ON MENDEL

How did Mendel determine the concept of dominant and recessive ‘factors?’

(What was his experiment)?

Question...

• Does one trait affect another?

(Does pea colour influence pea shape?)

Mendel’s Dihybrid CrossExperiments

• First produced plants that were purebred for 2 traits that he wanted to examine. – Ex// Plant with round yellow peas & Plant with

wrinkled, green peas.

These plants were HOMOZYGOUS for both traits.

Mendel’s Dihybrid CrossExperiments (Example)

• Mendel bred plants until the offspring always had round, yellow seeds. (RR YY)– Homozygous dominant

• He also bred plants with wrinkled, green seeds. (rr yy)– Homozygous recessive

Dihybrid Cross!

• F1 generation all had round, yellow seeds.– Why is this?

Dihybrid Cross!

• F1 generation all had round, yellow seeds.– Why is this?

– The offspring are all heterozygous for the two traits. (RrYy)

RRYY

RRYY

rryy

RRYY

rryy

RrYy RrYy

• Mendel allowed the F1 generation to self-pollinate. Of 551 plants in the F2 generation:– 320 round– 104 round green– 101 wrinkled yellow– 26 wrinkled green

FIND THE RATIO OF PHENOTYPES!

Law of Independent Assortment

• What explains the ratio?– If the alleles from one trait were inherited

independently of the alleles for another trait.• LAW OF INDEPENDANT ASSORTMENT

Law of Independent Assortment

• What explains the ratio?– If the alleles from one trait were inherited

independently of the alleles for another trait.• LAW OF INDEPENDANT ASSORTMENT

• Different pairs of alleles are passed to the offspring independently from each other.

WHAT DOES THIS MEAN!?

• Offspring may have new combinations of alleles that are not present in either parent.

Dihybrid Test Crossing

• Two-trait test cross involves crossing an individual that shows the dominant phenotype for the two traits with an indivudual that is homozygous recessive for the same two traits.

• The individual showing the dominant phenotype for both traits may be either heterozygous or homozygous dominant.

• Use a homozygous recessive individual to cross because better chance of producing an offxpring that is homozygous recessive for both traits.

Example:

• Ex//– Pea plant homozygous dominant for purple

flower colour (PP) and round for pea shape (RR), then the F1 generation will all have __________________ phenotypes.

– If heterozygous for both traits (PpRr), then 25% chance the F1 generation will show the recessive condition for one or both of the traits.

Sample Problem

• A male and female guinae pig are both heterozygous for fur colour and fur texture. Both dark fur (D) and rough fur (R) are dominant traits. a) What are the recessive traits and what variables would you

use for them?

b) What is are the parent’s phenotypes?

c) How many different gametes are formed and what are they?

d) Determine the frequency of offspring that are homozygous for both traits.

e) Determine the frequency of offspring that have rough, dark fur.

f) Determine the frequency of offspring that express both recessive traits.

• ICW: Practice problem, Page 140

• Section review, page 141: #1-9

4.4 – Beyond Mendel’s Laws

• Some organisms show different patterns of inheritance other than dominant and recessive.

INCOMPLETE DOMINANCE

• In some cases, neither of the alleles is dominant.

• Blending of traits can occur: incomplete dominance.

• Occurs in heterozygotes.

Example

• Snapdragons

– White and red flowers are

homozygous.- Pink flowers are

Heterozygotes.

• The variables R and R’ are used to indicate alleles.– Two R (R red) alleles are necessary to

make a red flower individuals with only one R allele are unable to make enough red pigment to produce red flowers.

– Two R’ (R’ white) alleles are necessary to make a white flower.

• If RR and R’R’ (P) flowers were to be crossed, what would be the genotypes of the offspring?

• If RR and R’R’ (P) flowers were to be crossed, what would be phenotypes of the offspring?